1. Field of the Invention
The present invention relates to a structure of a rotary electric machine and, more particularly, to a rotary electric machine capable of efficiently cooling a stator thereof and having excellent magnetic properties.
2. Description of the Background Art
A conventional AC motor includes a stator configured with a plurality of independent stator pieces on which stator coils are wound, the stator pieces being arranged in a ringlike structure, and a stator retaining ring having an opening in which the stator is fixed by press-fitting, wherein the stator retaining ring and a motor housing are fixed by screws. This kind of AC motor is described in Japanese Patent Application Publication No. 2001-025187, for example.
An example of a conventional rotary motor includes a cylindrical aluminum housing, a cylindrical iron sleeve fixedly arranged on a cylindrical inner surface of the housing, and a ring-shaped stator configured with a plurality of stator pieces made of a magnetic material that are fixedly arranged side by side along a circumferential direction on a cylindrical inner surface of the iron sleeve in close contact therewith. The iron sleeve is firmly fit in the housing by shrinkage fitting. Japanese Patent Application Publication No. 2003-284269 describes this kind of rotary motor.
Also, an example of a conventional cooled-casing-type rotary electric machine includes a stator frame in which a coolant passage is formed, the stator frame having an inner circumferential portion serving to retain a stator core. Japanese Patent Application Publication No. 1994-269143 describes this kind of rotary electric machine.
In the aforementioned conventional AC motor, the stator retaining ring and the motor housing are fixed by screws and there is formed a thick layer of air between an outer surface of the stator retaining ring and the motor housing. This AC motor therefore has a problem that the motor can not be sufficiently cooled when the temperature thereof has increased under operating conditions, because the stator and the stator retaining ring can not easily dissipate heat.
In the aforementioned conventional rotary motor, the iron sleeve is firmly fit in the housing by shrinkage fitting so that an outer surface of the iron sleeve and an inner surface of the housing are in close contact with each other. Therefore, the iron sleeve thermally expands when the rotary motor is run and the temperature thereof increases. As a consequence, contact pressure between the iron sleeve and the housing increases by compression, causing a compressive stress in the stator which is disposed in close contact with the inner surface of the iron sleeve. This develops a problem that iron loss in the stator increases under operating conditions, leading to deterioration of magnetic properties.
The aforementioned cooled-casing-type rotary electric machine has the stator frame in which the coolant passage is formed. Thus, the stator frame is cooled by a coolant while the stator thermally expands due to a temperature increase during operation of the rotary electric machine. For this reason, contact pressure between the stator frame and the stator remarkably increases under operating conditions and, as a consequence, there arises a problem that iron loss in the stator increases under operating conditions, leading to deterioration of magnetic properties.